United Airlines confirmed on Thursday that it will seek proposals from airplane makers Boeing Co. and rival Airbus for a major jet order that could be placed as soon as this fall.

The nation’s third-largest airline said it will look for proposals to replace its widebody fleet over several years, as well as its Boeing 757s.

Unlike other carriers that have at least kept their toes in the new-jet pool, United has held back on ordering new airplanes. It has been shrinking to try to match the flying it does with remaining demand, and it’s working on eliminating its Boeing 737 fleet altogether.

Glenn Tilton, chairman and chief executive of Chicago-based United, told employees in a message on Thursday that it has had ongoing discussions with both Boeing and Airbus over several years, but decided to hold off on new planes “until we believe we can generate a return on our investment.”

“The analysis we have conducted for more than a year suggests that time may be now,” Tilton said.

Copyright 2009 The Associated Press.